After completing extensive studies on spontaneous and induced tumors in nude mice, which showed no major differences in tumor incidence, type or biological behavior between the athymic nude mice and their controls, we plan to continue the work along two main lines: 1) to characterize possible thymus-independent surveillance mechanisms in the nude mouse to explain the "normal" incidence of tumors and 2) to determine if the natural cytotoxic cells detected in nude and normal mice in vitro, against targets derived from solid tumors (chemically induced, spontaneous, viral-induced mammary tumors, etc.) may have in vivo significance. These correlations will include attempts at in vivo abrogation of the thymus-independent putative surveillance mechanisms and determine the effects of such procedures on tumor development and biological behavior. To a large extent, this is a re-evaluation of the immune surveillance concept, but as a non-thymus-dependent proposition.